A display apparatus has a display panel. In the display panel, a liquid crystal is enclosed in between two transparent substrates, and, upon application of voltage, the orientations of liquid crystal molecules are changed with consequent variations in light transmittance, thereby permitting the display of a predetermined image or the like in an optical manner. In the display apparatus, since the liquid crystal is not a light emitter in itself, for example, the display panel of transmissive type has, at its back side, a backlight unit which is an illuminating device for applying light to the display panel using a light source such as a cold-cathode fluorescent tube (CCFL) or a light-emitting diode (LED).
Backlight units are classified into two categories, namely a direct-lighting type in which light sources such as cold-cathode fluorescent tubes or LEDs are arranged at the bottom for light emission, and an edge-lighting type in which light sources such as cold-cathode fluorescent tubes or LEDs are arranged at an edge portion of a transparent plate called a light guide plate, so that light can be directed forward, through printed dots or patterns formed at the back, from the edge of the light guide plate.
Although the LED has excellent characteristics, including lower power consumption, longer service life, and the capability of reduction in environmental burdens without the use of mercury, its use as a light source for a backlight unit has fallen behind because of its expensiveness, the fact that there had been no white-color LED prior to the invention of a blue-color LED, and its high directivity. However, in recent years, as white-color LEDs exhibiting high color rendition and high brightness spring into wide use for illumination application purposes, LEDs are becoming less expensive, and consequently, as a light source for a backlight unit, the shift from the cold-cathode fluorescent tube to the LED has picked up momentum.
LEDs have high directivity, wherefore a backlight unit of edge-lighting type has the advantage over a backlight unit of direct-lighting type from the standpoint of effecting light irradiation in a manner such that a display panel can exhibit uniform surface brightness in a planar direction. However, the edge-lighting type backlight unit poses the following problems: localized arrangement of light sources at the edge portion of the light guide plate results in concentration of heat generated by the light sources; and the size of the bezel portion of the display panel is inevitably increased. Furthermore, the edge-lighting type backlight unit is subjected to severe restrictions in terms of local dimming control which attracts attention as a control technique capable of display of high-quality images and energy saving, and is therefore incapable of split-region control that achieves production of high-quality displayed images and low power consumption as well.
In view of the foregoing, studies are going on to come up with a method whereby, even if a highly-directive LED is used as a light source in a direct-lighting type backlight unit having an advantage in its suitability for local dimming control, a display panel can be irradiated with light with uniformity in the brightness of the display panel in its planar direction. As a technology to achieve uniformity in brightness, for example, in Patent Literature 1, there is described an illuminating apparatus comprising: a box-shaped frame member which defines a single region to be illuminated in an object to be illuminated; a printed substrate disposed on the bottom surface of the frame member; and a light-emitting device surrounded by the frame member. The light-emitting device includes: a light-emitting element disposed on the printed substrate; a lens in which one part of light emitted from the light-emitting element passes through the upper surface thereof for its exit, and another part of the light passes through the side surface thereof for its exit; and a diffusely-reflecting sheet disposed in a location on the frame member other than the location where the printed substrate is mounted.